Method for removing iron impurities contained in a salt bath for nitrogenation

ABSTRACT

THE PRESENT INVENTION RELATES TO A PROCESS FOR SURELY AND ECONOMICALLY REMOVING IRON IMPURITIES CONTAINED IN A SALT BATH FOR NITRIZATION COMPRISING CYANIDE AND CYANATE BY ELECTROCHEMICALLY DEPOSITING THE IRON IMPURITIES ON A   CATHODE USING AN ANODIC AND A CATHODIC ELECTRODE PROVIDED IN THE SALT BATH.

Apnl 10, 1973 5E||Hl TA As E'AL 3,726,772

METHOD FOR REMOVING IRON IMPURITIES CONTAINED IN A SALT BATH FOR NITROGENATION Filed Feb. 2, 1971 2 Sheets-Sheet l United States Patent O 3 726 772 METHOD FOR REMOVING IRON IMPURITIES CONTAINED IN A SALT BATH FOR NITRO- GENATION Seiichi Takahashi, Takeshi Uehara, and Yoshialu Shimizu, Kyoto, Japan, assignors to Mitsubishi .Iidosha Kogyo Kabushiki Kaisha, Tokyo, Japan Filed Feb. 2, 1971, Ser. No. 111,906 Claims priority, application Japan, Feb. 9, 1970, 45/ 11,287 Int. Cl. C23c 9/14 U.S. Cl. 204-39 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a process for surely and economically removing iron impurities contained in a salt bath for nitrization comprising cyanide and cyanate by electrochemically depositing the iron impurities on a cathode using an anodic and a cathodic electrode provided in the salt bath.

BRIEF EXPLANATION OF DRAWING FIGS. l-3 are sectional views of an apparatus suitable to practising a process of the present invention for removing iron impurities and preventing an increasement of iron impurities, FIG. 1 being that suitable to practising a process of the present invention for intermittently effecting before or after nitrization and FIG. 2 being that suitable to practising a process of this invention for effecting in parallel with nitrization; FIG. 3 being that suitable to continuously practising thereof; FIGS. 4-6 are diagrams showing test results for removing the iron impurities according to the process of the present invention taking a treating time as abscissa and a degree of purification (calculated on a weight basis of sodium ferrocyanide) as ordinate.

THE DETAIL'ED EXPLANATION OF THE INVENTION The present invention relates to the process for removing the iron impurities contained in the salt bath for nitrization mainly used for impregnating steel products to be treated.

Generally, nitrization for metals, especially iron and iron alloy, is effected in a molten bath for nitrization namely Taftride Bath, trade name, consisting of 30-65% by weight of cyanide (calculated on the weight basis of NACN) and 15-45% by weight of cyanate (calculated on the weight basis of KACNO) and residue of alkali carbonate at a temperature of about 500600 C., preferably around 570 C., for 1.5-2 hrs. during blowing air or oxygen in the salt bath for nitrization.

It is well known that the iron impurities have a bad effect on an efiiciency for nitrization and make worse properties of the nitriding, layer formed on the products to be treated and make weaker the treating capacity of the salt bath for nitrization as the iron impurities content increases. Such phenomena is accelerated remarkably as the iron impurities contained in the molten salt bath increase, especially in case that the products to be treated is iron products or iron alloy products having a forged skin. In such a case, the step for removing the iron impurities contained in the salt bath may often account for about 30% of the entire nitriding process.

The maximum permissible concentration of the iron impurities in the salt bath is usually less than about 0.15% by weight calculated on the weight basis of sodium ferrocyanide Na Fe(ON) of course, the smaller the content of the iron impurities the better.

3,726,772 Patented Apr. 10, 1973 More than 0.15% by weight of the iron impurities dissolved in the salt bath effect a bad influence on the formation of the surface film formed.

The conventional process for removing the iron impurities contained in the salt bath usually comprises the following two steps which are both carried out by mechanical operations:

(1) The undissolved iron impurities in or floating on the surface of the salt bath are scooped up by a skimming means and then removed.

(2) The iron impurities dissolved in the salt bath mainly consisting of sodium ferrocyanide or other complicated iron complex salts are made insoluble by raising the bath temperature to 600630 C. resulting in the formation of e-lIOIl, but not sure, and then the precipitated material is removed mechanically in the same manner as in item (1).

However, these conventional methods require a long time for the treatment because of their mechanical removing operation, and have also poor reliability and in some cases, for example in the case of said item (2), the bath temperature is raised over the treating temperature so that it inevitably causes early wear of the lining material such as titanium on the inside of the salt bath or early ageing of the bath composition itself resulting in uneconomy of the lining material.

There are disclosed in German Pat. Nos. 608,257 and 1,255,438 well known electrochemical processes for nitrization and carbonitrization.

In the process disclosed in German Pat. -No. 608,257, nitrization must be electro-chemically efiected for long time by using iron or iron alloy products as the anode and the salt tank as the cathode at the temperature of less than 721 C.

Although non-porous solidity quality of very thin film layer is formed on the iron or iron alloy products in the beginning of the treatment, said layer formed will redissolve by an anodic effect during the long treating time.

In view of such condition, the treating temperature of more than 721 C. will be necessary to accelerate a diffusion of nitrogen into a difiusion area.

In the process disclosed in German Pat. No. 1,255,438, nitrization must be electrochemically effected by using the bath tank as the cathode and the auxiliary electrode as the anode.

(1) In case that cast iron salt bath is used, nitrization is electrochemically eifected using the salt bath as the cathode and titanium or aluminium electrode as the anode whereby depositing the iron impurities on the wall of the salt bath. Since the efiiciency of nitrization becomes bad as the iron impurities in the salt bath as well as those deposited on the wall of the salt bath increase, there is no way but exchanging salt bath. It is a disadvantage of the invention to redissol've the iron impurities deposited on the Wall of the salt bath by means of an interruption of electric current and a stop of circularization.

(2) In case that titanium salt bath is used, nitrization is eletrochemically eifected using the salt bath as the cathode and iron or iron alloy electrode as the anode thereby dissolving the wall of the salt bath when a voltage of more than a decomposition voltage is charged.

It is another disadvantage of the invention to limit an electric current used in the salt bath to for example less than 5 amp./dm.

In such situation, the cathode is easy to exchange, while the anode is difiicult to exchange. When the nitrization is continued without any exchange of the electrodes, the efficiency of the nitrization will decrease as that of the item 1).

The present invention resides in a process for removing the iron impurities with good efficiency by using easily removable electrodes and circulating a direct electric current between both electrodes provided in the salt bath for nitrization, whereby depositing the iron impurities contained in the salt bath on the surface of the cathode.

The realization of a process of the present invention is explained as follows.

FIGS. 1 and 2 illustrate an embodiment of a process for removing the iron impurities or preventing an increasement of the iron impurities contained in the salt bath tank according to the present invention, the process of FIG. 1 being effected before and after nitrization interminently and also the process of FIG. 2 being effected together with nitrization.

In FIGS. 1 and 2, the anode 3 and the cathode 4 are disposed in a molten salt 2 of the salt bath 1 for nitrization, and a direct electric current from an electric source E is applied to said electrode thereby to cause a deposition of the iron impurities on the surface of the cathode 4, the latter 4 being taken out of the bath from time to time to wash away the iron impurities deposited on the cathode in the course of treatment for a repeated use thereof.

The article 5 that is being subjected to nitriding may be continuously treated during circulating the electric current between the electrodes provided in the salt bath for removing the iron impurities or may be taken out of the salt Without any hindrance of nitrization.

FIG. 3 shows another embodiment of a continuous process for removing the iron impurities contained in the salt bath according to the process of the present invention wherein the salt bath 1 for nitrization and the tank 6 for removing iron impurities are provided separately, said both tanks being communicated with each other through conduits 7, 8, whereby the molten salts 2, 2' in said tanks are smoothly transferred and circulated by means of a pump 9 thereby performing the continuous nitrization treatment in which nitrization for the products 5 as well as the removal of the iron impurities or the prevention for increasing the iron impurities are conducted separately but simultaneously.

The actual capacity of the nitriding tank 1 increases in the comparison with that of FIG. 2 thereby the capacity for nitrization increases.

The anode 3 used in the process of the present invention is made of a material such as platinum of carbon rod, which is highly resistant to corrosion by the molten salt bath and does not contain any harmful impurities such as iron, but usually, a carbon rod for electrode is suitable for use.

The cathode 4 may be made of any suitable material having electro-conductivity property, but preferably, an iron-made wire gauze or the like which is easy to deposit the iron impurities on the surface thereof.

Both electrodes may be suitable configuration and arrangement if they are disposed opposite to each other so as to produce best workability.

The salt bath tank 1 and the iron removal treatment tank 6 in FIG. 3 are usually iron pots, so that it is indispensable matter to provide a lining of titanium material in the prior process to present undesirable radissolution of tank wall.

However, the present invention has an advantage of omitting such titanium lining.

For the electric current, DC-source may be used, but a pulsating current or a half-wave rectified current obtained by rectifying an alternate current through a selenium rectifier may be used. The DC voltage applied is usually less than 40 volts.

If such voltage is too high, it results a depression of efficiency of a deposit of the iron impurities on the surface of the cathode, while too low voltage will also invite a reduction in an operating efiiciency. Thus the most suittable voltage range is about 1 to 10 volts,, while the current density used is less than about 20 amp./dm. preferably less than about 10 amp./dm.

Example of a process for removing iron impurities from the salt bath will now be given as follows:

4 Example 1 The electrodeposition was effected in the molten salt bath (7504 x 1600 mm.) comprising 48% by weight of potassium cyanide and 43% by weight of potassium cyanate as shown in FIG. 1 using the carbon rod (50 mm. dia x 750 mm. length) as the anode and a cylindrical wire gauze (430 mm. dia x 930 mm. length) as the cathode and handing the product to be treated between the anode and the cathode provided in the molten salt bath in order to remove the iron impurities contained in the salt bath.

An initial concentration of the iron impurities was about 0.2% by weight calculated on the weight basis of sodium ferrocyanide.

The test result as shown in FIG. 4 was obtained using the current density of l amp./dm. and an electric current of 2.5 volts and amperes.

The iron impurities decreased to 0.07% after treating for 60 minutes.

Example 2 The electrodeposition was effected using the same molten salt bath and the treating conditions as shown in the Example 1. The initial concentration of the iron impurities was about 0.16% by weight calculated on the weight basis of sodium ferrocyanide.

The test result as shownin the FIG. 5 was obtained using the current density of 2.5 amp./dm. and the electric current of 6.0 volts and 250 amperes.

The iron impurities decreased to 0.05% after treating for 30 minutes.

The treating efficiency for nitrization increases by about 30% in comparison with that in case of a prior mechanical removing process using same bath in capacity.

Example 3 The electrodeposition was effected using the same molten salt bath and the treating conditions as shown in the Example 1 using the apparatus (800 mm. dia x 1600 mm. height) as indicated in the FIG. 2 and six carbon rods (26 mm. dia x 700 mm. length) as the anode and the cylindrical iron wire gauze (700 mm. dia x 930 mm. length) as the cathode charging goods to be treated every 2 hrs. continuously. The goods to be treated are made of cast iron. The total weight amounts to kg. The initial concentration of the iron impurities was about 0.08% by weight calculated on the weight of sodium ferrocyanide.

The test result as shown in FIG. 6 was obtained using the current density of 0.4 amp/dmfi.

The continuous operation for 24 hours was effected by charging eleven times of the products to be treated without any increasement of the iron impurities.

According to the process of the present invention, the following advantages are obtained.

(1) As the undissolved iron impurities in the salt bath are deposited on the surface of the cathode in the form as they are, and the dissolved iron impurities are also deposited on that of the cathode, the removal of the iron impurities may be effected by removing the iron impurities deposited and accumulated on the cathode at an interval of the treating time.

(2) As the principle of so-called molten salt electrolysis or electrodeposition is employed in the process of this invention, automatic control system is made possible whereby the control of the concentration of the iron impurities can be performed with higher reliability and easier than in case of an ordinary mechanical operation.

(3) According to the ordinary mechanical operation, nitrization must be suspended in the iron removal process, whereas the process of the present invention makes possible the continuous operation thereby remarkably reducing the number of the steps required for removing the iron impurities.

(4) The iron impurities dissolved in the salt bath may be removed electrochemically without elevating the bath temperature, so that there is no fear of inviting excess reduction of a service life of the pot lining or early aging of the bath composition as observed in the conventional process.

(5) The electrode is easy to exchange since the salt bath is not used as the electrode, while the efiiciency for removing the iron impurities is high as the current applied is not limited.

What is claimed is:

1. A process for removing iron impurities contained in a molten salt nitriding bath that is maintained at a temperature of from about 500 C. to about 600 C. including the steps of disposing a removable cathode in contact with the molten salt bath and causing a unidirectional electric current to flow between an anode and said cathode thereby to deposit on the cathode iron impurities contained in the salt bath.

2. A process according to claim 1 wherein the cathode is periodically removed from the salt bath thereby to remove the electro deposits of iron on the cathode from the salt bath.

3. A process according to claim 2 wherein the accumulated iron deposits are removed from the cathode while the cathode is out of the bath and the cathode is then replaced in the bath.

4. A process as claimed in claim 1 wherein the removal of iron impurities is eifected employing a current density of less than 20 amp/dm. and a voltage of less than 40 volts.

5. A process as claimed in claim 1 wherein the removal of iron impurities from the molten salt bath is effected by the use of a current density of less than 10 amp/dm. and a voltage of from 1 to 10 volts.

6. A process as claimed in claim 1 wherein the unidirectional current is provided by means of a pulsating direct current.

7. A process as claimed in claim 1 wherein the unidirectional current is obtained by means of a half wave rectified alternating current.

8. A process according to claim 1 wherein the cathode is immersed in a portion of the molten salt nitriding bath contained in a container separate from the bath in which the article to be nitrided is disposed, and the molten salt bath is circulated between the container containing the cathode and the container in which the article is disposed.

References Cited UNITED STATES PATENTS 1,685,156 9/1928 Giordani 204--112 X 1,929,252 10/1933 Morris 204-39 X 2,057,274 10/1936 Mayhew 204-39 X 2,820,7 1/ 195 8 Von Bichowsky 204-39 X JOHN H. MACK, Primary Examiner A. C. PRESCOTT, Assistant Examiner US. Cl. X.R. 

